In the sewn goods industry, where various sections of material are sewn together to fabricate products, precise seam lengths and end points are often necessary for proper appearance and function of the finished products. For example, the top stitch seam of a shirt collar must closely follow the contour of the collar and terminate at a precise point which matches the opposite collar. Accurate seam lengths must similarly be maintained in the construction of shoes when sewing together vamps and quarter pieces to achieve strength as well as pleasing appearance. Achieving consistently accurate seam lengths and end points at high rates of production has, however, been a long standing problem in the industry.
Microprocessor controllers have been developed which convert manually operated sewing machines into semi-automatic sewing systems. U.S. Pat. Nos. 4,108,090; 4,104,976; 4,100,865; and 4,092,937, assigned to the Singer Company are representative of such devices. Each of those patents discloses a programmable sewing machine with three operational modes: manual, teach and auto. Control parameters are programmed into the system for subsequent control of the sewing machine in the auto mode. Those microprocessors control all sewing machine functions such as sewing speed, presser foot position, thread trimmer, reverse sew mechanism and the number of stitches sewn in each individual seam. Accurate control of seam lengths is one of the important aspects of those systems.
U.S. Pat. No. 4,404,919, issued Sept. 20, 1983, entitled "Control System for Providing Stitch Length Control of a Sewing Machine", and assigned to assignor describes a microprocessor controlled sewing system which improves upon the seam length accuracy of those systems. The described system controls seam length accuracy using a combination of stitch counting, edge detection and stitch length control techniques. Control of seam lengths and end points is achieved in the system by initiating countdown of a variable number of final whole and partial stitches responsive to detection of the end of the material being sewn by sensors located ahead of the needle. Though the system disclosed in U.S. Pat. No. 4,404,919 provides improved accuracy over the previous systems which relied solely upon stitch counting to determine seam lengths and end points, the system does not account for variations in stitch length with respect to sewing speed. It is known, for example, that stitches sewn at higher speeds are generally longer than stitches sewn at lower speeds. Thus, in a system where a combination of edge sensing and stitch counting is used to accurately control seam margins, stitch length changes due to speed variances may nonetheless cause inaccurate results.
A need has thus arisen for an improved adaptive sewing machine control system which includes a stitch length control technique which compensates for sewing speed.